An operational amplifier for a grayscale power supply generally has five amplifiers on the positive side and five on the negative side if it is a 6-bit operational amplifier, and nine amplifiers on the positive side and nine on the negative side if it is an 8-bit operational amplifier. These amplifiers are designed to be capable of producing an output up to the vicinity of the power-supply potential or ground potential, taking into consideration the efficiency of the power supply.
Grayscale power supplies are used frequently in special-purpose ICs but there are also cases where they are incorporated in LCD (Liquid Crystal Display) drivers. In such cases there is little leeway in terms of driving capability because the amplifiers are of CMOS construction. Improvements in terms of circuitry, therefore, are required.
FIG. 4 is a diagram illustrating the structure of an ordinary LCD source driver and LCD panel according to the prior art. The LCD source driver includes a data register 1 that accepts digital display signals R, G, B or six bits each; a latch circuit 2 for latching a 6-bit digital signal in sync with a strobe signal ST; a D/A converter 3 comprising N stages of parallel-connected digital/analog converters; a liquid-crystal grayscale voltage generating circuit 4 having a gamma (γ) conversion characteristic that conforms to the characteristic of the liquid crystal; and N-number of voltage followers 5 for buffering voltage from the D/A converter 3.
The LCD panel includes thin-film transistors (TFTs) 6 provided at the intersections of data lines and scanning lines, each transistor having its gate connected to a scanning line and its source connected to a data line; and pixel capacitors 7 having one end connected to the drain of the corresponding TFT and its other end connected to a common terminal COM.
In the LCD panel shown in FIG. 4, N-number of TFTs are provided in each of M-number of rows, although only one row is illustrated in FIG. 4. An LCD gate driver (not shown) drives the gates of the TFTs of each line one after another. The D/A converter 3 converts a 6-bit digital display signal from the latch circuit 2 to analog signals and supplies these to the N-number of voltage followers 5-1 to 5-N. The resultant signals are applied to liquid crystal elements, which act as the pixel capacitors 7-1 to 7-N, via the TFTs 6-1 to 6-N.
Reference voltages are generated by the liquid-crystal grayscale voltage generating circuit 4, and a selection of reference voltage is made by a decoder implemented by a ROM switch (not shown), etc., in the D/A converter 3.
The liquid-crystal grayscale voltage generating circuit 4 incorporates a resistance ladder circuit (not shown). The output is driven by a voltage-follower arrangement in order to lower the impedance of each reference-voltage tap and in order to finely adjust the reference voltage.
FIG. 5 is a diagram illustrating the structure of a liquid-crystal grayscale voltage generating circuit for driving a resistance ladder circuit by a voltage follower (see Japanese Patent Kokai Publication Nos. JP-A-6-348235 and JP-A-10-142582). In FIG. 5, the grayscale voltage generating circuit includes a resistance ladder circuit 10 (resistors R1, R2, . . . , Rn−2, Rn−1) provided internally of an LCD driver; an external resistance ladder circuit 30 (resistors R01′, R1′, R2′, . . . , Rn−2′, Rn−1′); a buffer amplifier 20 (operational amplifiers OP1, OP2, . . . , OPn−1, OPn) comprising a voltage follower for outputting reference voltages V1 to Vn upon receiving tap voltages from the external resistance ladder circuit 30 as inputs; and a constant-voltage generating circuit 40 (Vr). The ladder resistors R01′, R1′, R2′, . . . , Rn−2′, Rn−1′ of the external resistance ladder circuit 30 are variable resistors and regulate the voltages applied to the operational amplifiers OP1, OP2, . . . , OPn−1, OPn of the buffer amplifier 20. The regulated voltages are adjusted so as to be best suited to the characteristics of the liquid crystal panel.
The reference supply voltages in the liquid-crystal grayscale voltage generating circuit of FIG. 5 are ground potential GND and Vr. The reference supply voltage Vr is applied by the stable external constant-voltage generating circuit 40 such as a band-gap reference circuit. Grayscale voltages Vn, Vn−1, Vn−2, . . . , V2, V1 are finally decided by the ladder resistors R01′, R1′, R2′, . . . , Rn−2′, Rn−1′.
More specifically, we have the following:Vn=Vr Vn−1=Vr{(Rn−2′+Rn−3′+ . . . +R0′)/(Rn−1′+Rn−2′+Rn−3′+ . . . +R0′)}
Similarly,V1=Vr{R0′/(Rn−1′+Rn−2′+Rn−3′+ . . . +R0′)}
If each resistance ratio of the ladder resistors R1, R2, . . . , Rn−2, Rn−1 that decide the grayscale voltages internally and each resistance ratio of the ladder resistors R01′, R1′, R2′, . . . , Rn−2′, Rn−1′ that decide the grayscale voltages externally are the same, then the output currents of the operational amplifiers OP2, OP3, . . . , OPn−1 will be zero.
However, the output current In of an nth operational amplifier OPn (the operational amplifier whose output has the highest potential) counting from the ground side is given by Equation (1) below in the source direction.In=(Vn−V1)/(R1+R2+ . . . +Rn−1)=I0  (1)
The output current I1 of the first operational amplifier OP1 (the operational amplifier whose output has the lowest potential) counting from the ground side is given by Equation (2) below in the sink direction.I1=(Vn−V1)/(R1+R2+ . . . +Rn−1)=I0  (2)
Thus, a problem which arises in the liquid-crystal grayscale voltage generating circuit shown in FIG. 5 is that the output dynamic range of the operational amplifiers OPn, OP1 diminishes owing to the source-direction output current In of operational amplifier OPn and sink-direction output current I1 of operational amplifier OP1 indicated by Equations (1) and (2).
In order to solve this problem, the applicant proposes arrangements of the kind shown in FIGS. 6A, 6B or in FIGS. 7A, 7B in Japanese Patent Application Kokai Publication No. JP-A-10-142582.
Specifically, as shown for example in FIG. 6A, an auxiliary resistor Rn is connected between a high-voltage power-supply terminal VDD and ladder resistor Rn−1, and an auxiliary resistor R0 is connected between a low-voltage power-supply terminal GND and ladder resistor R1. Other components are similar to those shown in FIG. 5. By virtue of such an arrangement, source current of the voltage follower OPn on the side of the high-voltage power-supply terminal VDD is adjusted by the resistor Rn, and sink current of the voltage follower OP1 on the low-voltage power-supply terminal GND is adjusted by the resistor R0. It should be noted that FIG. 6B is constructed by removing the resistor Rn/2 in the internal resistance ladder of FIG. 6A.
Further, as illustrated in FIG. 7A, auxiliary current sources I0, In are connected instead of the auxiliary resistors R0, Rn. Here it is assumed that the auxiliary current sources I0, In are set so as to satisfy Equations (1), (2). According to this arrangement, the source current and sink current of the operational amplifiers OPn, OP1 become zero, the output dynamic range is broadened and it is easier to design the output stages of these operational amplifiers. It should be noted that FIG. 7B is constructed by removing the resistor Rn/2 in the internal resistance ladder of FIG. 7A.
FIG. 8 illustrates the connections between buffer operational amplifiers AH, AL, which construct the grayscale power-supply circuit, and γ resistors (grayscale resistors for γ adjustment) of a plurality of LCD drivers. Wiring resistors serving as parasitic resistance of wiring connecting the plurality of LCD drivers are shown in FIG. 8 in terms of a circuit diagram. That is, γ resistors from the first LCD driver to the nth LCD driver are connected in parallel. Furthermore, nodes connected to the maximum and minimum potentials of the γ resistors are connected to the outputs of the buffer operational amplifiers, but parasitic resistance components (wiring resistances are produced in the wiring connecting the γ resistors in parallel.
As shown in FIG. 8, wiring resistance components are produced in regular order, namely between the γ resistor of the first LCD driver (first driver) and the γ resistor of the second LCD driver (second driver), . . . , and between the γ resistor of the (n+1)th LCD driver and the γ resistor of the nth LCD driver (nth driver).
Thus, in the conventional LCD drivers, adopting the implementations of FIGS. 6A, 6B and 7A, 7B has the effect of widening output dynamic range and facilitating the designing of the output stages of the operational amplifiers. However, the ordinary LCD driver is not used only at a certain constant voltage that has been decided, and in most cases the voltage value used differs for every manufacturer of LCD modules. In general, therefore, a certain range of voltages (e.g., VDD2: 8 to 13.5V) is stipulated in the specifications of LCD drivers and operation within this range of power-supply voltages is assured.
Thus, if the power-supply voltage is subjected to variations, then the current that flows into the γ resistors also varies as a matter of course. As a consequence, the value of the constant-current auxiliary current source connected to the γ resistors and the value of the current that flows into the γ resistors will not exactly coincide.
This means that the difference between the value of the constant-current auxiliary current source connected to the γ resistors and the value of the current that flows into the γ resistors flows into the output of the operational amplifier connected to the side of the highest potential or to the side of the lowest potential (if the difference current value is zero, no current flows into the output of the operational amplifier, as described above). Thus, there is only one point of a certain power-supply voltage where the output current of the operational amplifier for the grayscale power supply becomes zero.
For example, in a COG (Chip On Glass) panel-type device of recent interest, the above-mentioned wiring resistance component becomes as large as several hundred ohms at times. If wiring of γ resistors is performed under this condition, then, in the event that the output currents of the operational amplifiers AH, AL for the grayscale power supply are not zero, the γ characteristic of each LCD driver will differ owing to voltage drops caused by the output currents of the operational amplifiers AH, AL of the wiring resistors. This causes a display problem referred to as “block unevenness”.
In the case of a COG device, wiring resistance is great and the wiring resistance components between the γ resistors of the LCD drivers shown in FIG. 8 are so large that they cannot be ignored.